The invention is concerned with analytical technology and, more specifically, with the detection of a fluorescent species or fluorophore in a sample.
Fluorescent species or fluorophores emit fluorescent radiation when suitably stimulated by stimulating radiation. The emitted radiation can be used for chemical/biological analytic purposes, e.g. in determining whether a fluorophore of interest is present in a sample and in quantifying its concentration. One analytic technique of this type is disclosed in U.S. Pat. No. 5,171,534 to Smith et al. wherein DNA fragments produced in DNA sequencing are characterized on the basis of fluorescence of chromophores tagged to the fragments. Stimulating electromagnetic radiation may be monochromatic, or may include significant energy in a plurality of energy bands, e.g. as disclosed in U.S. Pat. No. 5,784,157 to Gorfinkel et al.
The stimulating radiation usually varies in time, either stochastically or regularly. Regular variation of the radiation intensity can be introduced artificially by modulating the intensity of the radiation source or the transmittance or reflectance of a filter element in the optical path. Regularly modulated radiation may be termed as encoded radiation if the temporal variation of the radiation is used as a carrier of information. Associated with such encoded radiation is a temporal code, i.e. a time-domain function which corresponds to the temporal evolution of the intensity of modulated radiation. A time-domain function can be formed as a linear combination of several suitable functions whose respective contributions to the linear combination can be quantified reliably. Suitable in this respect are sinusoidal functions of time, for example, oscillating at distinct frequencies.
In prior-art techniques, the encoded radiation is considered as continuous, with the time dependence of detected radiation intensity regarded as a continuous time-domain function.
Further background includes several known single-photon detection techniques for which W. R. McCluney, Introduction to Radiometry and Photometry, Artech House, 1996, pp. 114-122 provides a general introduction. Such techniques are designed for measuring modulated radiation, and they can be classified into two groups: (a) asynchronous photon counting and (b) synchronous detection. As described in Alan Smith, Selected Papers on Photon Counting Detectors, SPIE, Vol. MS 413, 1998, methods (a) of asynchronous photon counting involve the detection of a number of photons during a fixed time interval, e.g. one second, called the registration interval. These methods allow the determination of an average frequency of photon arrival. This frequency varies in time, either stochastically or regularly, and synchronous counting can be employed to measure the time variation. An essential limitation of this method is associated with the impossibility of measuring frequencies of modulation that are higher than the repetition rate of registration intervals. This difficulty is inherent in the principle of asynchronous counting, which is to keep track of the total number of photons received during the registration interval rather than register their times of arrival. A difficulty arises when the highest frequency fmod in the modulation spectrum of modulation radiation is comparable to or higher than the average frequency fphot of single-photon detection. In this case, if the frequency limit is increased by reducing the time interval chosen for counting, the technique becomes increasingly inefficient because the counter will count nothing during most registration intervals.
Methods (b) of synchronous detection involve measurement of the time of arrival of incident single photons. This time may be referenced to an xe2x80x9cabsolutexe2x80x9d clock, or may be measured relative to or xe2x80x9csynchronously withxe2x80x9d a triggering excitation signal. The triggering signal may be associated with the arrival of the first of detected photons, for example. Such methods are particularly valuable for application to fast processes, e.g. the fluorescent decay of a single excited dye molecule as described, e.g., by D. Y. Chen et al., xe2x80x9cSingle Molecule Detection in Capillary Electrophoresis: Molecular Shot Noise as a Fundamental Limit to Chemical Analysisxe2x80x9d, Analytical Chemistry, Vol. 68 (1996), pp. 690-696, typically requiring special electronics for handling fast temporal variations. An essential limitation of these methods is associated with the difficulty of maintaining records of high temporal resolution for a relatively long time. Thus, detecting photon arrivals at the temporal resolution corresponding to nanosecond time intervals over a one-second period requires acquisition of a billion data records. This makes methods of synchronous detection difficult to apply to the photometry of relatively slowly varying modulated single-photon fluxes.
We have recognized that, in detecting a fluorescent species in a sample, single-photon counting can be used to advantage, especially at low levels of fluorescent signal energy. Preferred detection techniques include methods in which (i) fluorescence-stimulating radiation is intensity-modulated in accordance with a preselected code, (ii) wherein it is the fluorescent radiation which is intensity-modulated with the preselected code, and (iii) wherein modulation with a preselected code is applied to a sample to influence a property, e.g. temperature, pressure, or an electric or magnetic field strength or frequency which functionally affects emitted fluorescent radiation.
Preferably, for registration of the signals from a sensing element of a single-photon detector, time of arrival is recorded, optionally in conjunction with registration of time intervals. Advantageously, in the interest of minimizing the number of pulses missed due to close temporal spacing of pulses, D-triggers can be included in counting circuitry.
The preferred techniques are generally applicable to photometry of time-encoded single-photon or particle fluxes. They involve measurement of time intervals between single-photon/particle arrivals combined with data analysis that permits decoding of the encoded radiation, i.e., discrimination between alternative possible codes and quantification of different combinations of mixtures of the codes. The techniques provide for the time intervals between successive pulses to be measured asynchronously, without requiring an external clock reference or special triggering signal. They provide for efficient measurement and decoding of time-encoded single-photon or particle fluxes.